1. Field
Aspects of the present invention generally relate to a sheet processing apparatus and an image forming apparatus, and particularly to a configuration that improves alignment of sheet bundles discharged on a stack tray after each of the sheet bundles is bound.
2. Description of the Related Art
Recently, in an image forming apparatus such as a copying machine, a printer, and a facsimile machine, a print system has been widely used in which a sheet processing apparatus is connected to the image forming apparatus. The sheet processing apparatus performs various processing such as binding processing, punching processing, and sorting processing on image-formed sheets. In such a sheet processing apparatus, a metal staple is generally used in the binding processing. When the metal staple is used, sheets are bound with the staple projecting from a surface of the sheet bundle.
However, when bound sheet bundles are sequentially stacked on a stack tray, there are cases where a staple of the sheet bundle already stacked on the stack tray catches on a staple of the sheet bundle to be discharged next, causing deterioration in sheet bundle alignment on the stack tray. Accordingly, U.S. Pat. No. 5,797,596 discusses a technique in which a binding position of a sheet bundle is changed when the predetermined number of copies is bound each time.
For example, as illustrated in FIG. 23, when an odd-numbered sheet bundle SA is bound, a stapling unit is moved by a distance Y1, Y2, or Y3 from a home position H to drive a staple 1000 in. When an even numbered sheet bundle SA is bound, the stapling unit is moved by a distance Y1−d, Y2−d, or Y3−d to drive the staple 1000 in. This movement distance difference d corresponds to a dimension of the staple 1000 in a width direction perpendicular to a sheet conveyance direction, the dimension of the staple 1000 being provided after the staple 1000 is driven. Consequently, whenever the predetermined number of copies is bound, the binding position is changed in such a manner that bound portions are not overlapped in one location on a stack tray, thereby enhancing alignment of the bound sheet bundles SA.
However, in such a conventional sheet processing apparatus, a change in the binding position for each predetermined number of copies causes sheet bundles to have different binding positions. This generates a plurality of different types of products in spite of the same job. Meanwhile, a position of the stapling unit needs to be shifted to shift the binding positions so that staples have a non-interference relationship with one another. The position of the stapling unit needs to be shifted by an amount greater than a dimension of the staple in a width direction in consideration of deviation in the amount of skew generated when sheet bundles are discharged or an amount of displacement generated when sheet bundles fall to the stack tray.
Consequently, a change in binding position can solve a situation where one staple catches on another. However, when bound sheet bundles are sequentially stacked on the stack tray, there are cases where a projecting portion of the staple of the sheet bundle already stacked on the stack tray catches on an edge of a sheet bundle discharged next. In this case, these sheet bundles are stacked on the stack tray with edge surfaces thereof misaligned and skewed, causing deterioration in alignment of the sheet bundles on the stack tray.
In addition, there are cases where a sheet bundle discharged next may pass over a staple 1000 of an already stacked sheet bundle with inertia without being stuck by the staple 1000 even when the next sheet bundle catches on the staple 1000. In this case, when a sheet edge of the next sheet bundle passes over the staple 1000, an area of the sheet edge is curled or scratched by the staple 1000. Such damage to the sheet edge area causes deterioration in product quality.